This invention relates to multiplexer circuits and to arrays of device elements arranged in rows and columns, and having column multiplexer circuits. In particular, the device elements are image sensor elements of a large area image sensor.
It is known to provide row or column multiplexers for addressing or reading an array of electronic devices. Such multiplexer circuits generally comprise an array of switches which selectively couple one of the switch inputs to a common output.
Various types of switch are available which may be used to form multiplexer circuits. One type of switch involves the use of a diode bridge having four diodes, the four connection points around the bridge defining an input, output and two control terminals. The control terminals enable the four diodes to be switched between forward bias, when the input is linked to the output, and reverse bias, when the output is isolated from the input. The use of such a switch as a voltage switching element is disclosed, for example, in the following documents:
U.S. Pat. No. 4,150,256 (e.g. FIG. 3) discloses a multiplexer circuit comprising an array of switches, each switch comprising the diode bridge having four diodes which are selectively forward biased or reversed biased by the application of control voltages to two control terminals. The two control terminals are connected to opposite ends of a transformer secondary winding. In this way, a current supplied to the diode bridge by the transformer winding is equal to a current drawn from the diode bridge. The circuit suffers from the drawback that a transformer must be provided for each switch, and the circuit is therefore not suitable for integration onto a large area circuit substrate.
U.S. Pat. No. 4,518,921 discloses a sample and hold switch in which the diode bridge is turned on and off by respective pairs of current sources. Again, these ensure that the currents supplied to and withdrawn from the diode bridge are equal. The sample and hold switch of U.S. Pat. No. 4,518,921 has an output capacitor so that the output voltage may rise or fall to follow the input voltage, when the diode bridge switch is in the ON state.
In order for the diode bridge to function correctly in transferring the input voltage to the output voltage, the voltages at the control terminals of the diode bridge must not be fixed, and in the above cases a transformer secondary winding may be used (which provides a floating voltage) or current sources may be provided (which provide a constant current independently of the voltage across them).
U.S. Pat. No. 3,059,228 discloses another use of a diode bridge sampling switch in which the control terminals of the diode bridge are allowed to float in dependence upon the input voltage, through the use of a diode and resistive network. The network includes two voltage source pairs for controlling the switching ON and OFF of the diode bridge. The switch functions to transfer an input voltage through the switch to the output terminal, which has a charge storage element enabling the output voltage to rise or fall following the input voltage. When the switch is turned ON, resistive feed from high voltage sources ensures that substantially constant and equal currents are supplied to and drained from the diode bridge.
The present invention seeks to provide a multiplexer circuit for an array of electrical devices using diode bridge switches which may be integrated, together with their respective supply circuitry, onto the substrate of the array of electronic devices, and which multiplexer circuit may be used to switch a current from the input of the diode bridge to the output. The prior art discussed above does not address the particular problems associated with integration of the circuit onto an array of electrical elements, nor with the use of the diode bridge as a current switching device.
For the current at the output of the diode bridge to be equal to the input current, the current supplied to the diode bridge by the control circuitry must be equal to the current withdrawn from the diode bridge. This follows from the basic principle that the total current flowing into the circuit must equal the total current flowing out of the circuit (Kirchhoff's current law). Therefore, if the diode bridge circuit is to be used as a current switch, it would be expected that either the supply terminals of the diode bridge must be coupled together, for example in the manner shown in U.S. Pat. No. 4,150,256, or that equal current sources must be provided, for example in the manner shown in U.S. Pat. No. 4,518,921.